Ah I think I get it now. So when you try to find a chain, you start with the hash. Let's assume the hash is 100. So then you try 200 as prime origin. If it doesn't work, you try 300. Or maybe you start with 234*100. You always start with a prime origin which can be divided by the hash.

Because now I did some calculations, too. My fallacy was that I thought there are so many big numbers which can be divided by the hash. But now I realize, it is only every 2^256th number. And that is not much...

Ok, thank you guys But that is good news for me, because if I was right this would be a problem.